**1. Introduction**

76 Biogas

Yang, F. ; Bick, A. & Shandalov, S. (2009). Yield stress and rheological characteristics of

83-90.

activated sludge in an airlift membrane bioreactor. *Jour Membrane Sci,* Vol.334, pp.

The evolution of modern technology for olive oil extraction has affected the industrial sector depending directly on the by-products obtained. The traditional three-phase continuous centrifugation process for olive oil extraction was introduced in the 1970s, notably to increase the processing capacity and extraction yield and to reduce labour. This three-phase manufacturing process of olive oil usually yields an oily phase (20%), a solid residue (30%) and an aqueous phase (50%), the latter coming from the water content of the fruit, which is usually defined as vegetation water. Such water, combined with that used to wash and process the olives, make up the so-called "olive mill wastewater" (OMW) and also contains soft tissues from olive pulp and a very stable oil emulsion (Borja et al., 2006). This process generates a total volume of traditional OMW of around 1.25 litres per kg of olives processed. Consequently, the three-phase centrifugation process caused an increase in the average mill size, a decrease in the total number of mills, increased water consumption and increased production of wastewaters.

The OMW composition is not constant either qualitatively or quantitatively and it varies according to cultivation soil, harvesting time, the degree of ripening, olive variety, climatic conditions, the use of pesticides and fertilizers and the duration of aging. The three-phase OMW is characterized by the following special features and components: intensive violetdark brown to black in colour; specific strong olive oil smell; high degree of organic pollution (chemical oxygen demand –COD– values up to 220 g/L); pH between 3 and 6 (slightly acidic); high electrical conductivity; high content of poly-phenols (0.5-24 g/L) and high content of solid matter (Niaounakis and Halvadakis, 2004).

The annual OMW production of Mediterranean olive-growing countries is estimated to ranging from 7 million to over 30 million m3. This huge divergence of results can partly be explained by the fact that the production of olives varies from one year to another due to weather conditions and plagues that can affect the olive trees. The average total production amounts approximately to 10-12x106 m3 per year and occurs over a brief period of the year (November-March). Spain produced 20% of the OMW of the Mediterranean basin (2-3x106

Influence of Substrate Concentration on the Anaerobic

considerably.

the greater weight produced.

residual oil difficult and expensive.

**2.1 The two-phase Olive Mill Solid Waste (OMSW)** 

ash with a 30% potassium content (Alba et al., 2001).

and other pigments (0.5%) (Borja et al., 2002).

the Andalusia Community (Borja et al., 2002).

also reduced as a result of the lower processing quantity.

higher oxidation stability and better organoleptic characteristics.

In addition, the disadvantages of two-phase manufacturing process are:

difficult –it can not be piled and must be kept in large ponds.

Degradability of Two-Phase Olive Mill Solid Waste: A Kinetic Evaluation 79

 The throughput of the two-phase centrifuge in relation to the oil quantity is higher because no additional water is required to produce the pulp. Energy consumption is

Oil produced by the two-phase centrifuge is of higher quality; in particular, it has

The operating costs are lower. Water utilization in the olive mill decreases

 The two-phase process, although it produces no olive mill wastewater as such, generates the wash waters derived from the initial cleansing of the fruit and from the purification of virgin olive oil. In addition, it combines the olive vegetation water that is generated with the solid waste to produce a single effluent stream in semi-solid form. This doubles the amount of "solid" waste (OMSW or 'alperujo') requiring disposal, and it cannot be composted or burned without some form of expensive pre-treatment. Two-phase OMSW has a moisture content significantly higher than that of traditional cake from three-phase centrifuges. This increased amount of moisture, together with the sugars and fine solids that in the three-phase system were contained in OMW give two-phase OMSW a doughy consistency and makes transport, storage and handling

Two-phase OMSW is characterized by higher values of the pulp/stone ratio, as well as

 This two-phase technology transfers the problem of disposing of the olive-mill waste from the mill to the seed-oil refineries. Two-phase OMSW, prior to oil solvent extraction, must be dried with considerably higher energy requirements than in the three-phase continuous oil production process, making the industrial recovery of the

The characteristics of two-phase OMSW are obviously very different from the characteristics of olive cake resulting from three-phase centrifuge systems. Two-phase OMSW is a thick sludge that contains pieces of stone and pulp of the olive fruit as well as vegetation water. It has a moisture content in the range of 60-70% while olive cake from a three-phase extraction process has only around 40-45% moisture. It also contains some residual olive oil (2-4%), 2%

The average composition of the two-phase OMSW is: water (60-70%), lignine (13-15%), cellulose and hemicellulose (18-20%), olive oil retained in the pulp (2.5-3%), mineral solids (2.5%). Among their organic components, the major ingredients are as follows: sugars (3%), volatile fatty acids (C2-C7) (1%), poly-alcohols (0.2%), proteins (1.5%), poly-phenols (0.2%)

As it can be seen, the two-phase OMSW has a high organic matter concentration giving an elevated polluting load. The high polluting power and large volumes of solid waste generated (around 2 millions of tons per year in Spain) can pose large-scale environmental problems, taking into account the 2000 Spanish olive oil factories, most of them located in

m3/year) before the implantation of the two-phase extraction process in most of the Spanish olive oil factories, which represented an equivalent pollution of 10-16x106 inhabitants in the short milling period (Nioaunakis and Halvadakis, 2004).

The efforts to find a solution to the OMW problem are more than 50 years old (Borja et al., 2006). There are many different types of processes that have been tested: detoxification processes (such as physical, thermal, physicochemical, biological and combination of processes), recyclying and recovery of valuable components, production system modification, etc. However, none of the detoxification techniques on an individual basis allow the problem of disposal of OMW to be solved to a complete and exhaustive extent, effectively and in an ecologically satisfactory way. At the present state of OMW treatment technology, industry has shown little interest in supporting any traditional process (physical, chemical, thermal or biological) on a wide scale. This is because of the high investment and operational costs, the short duration of the production period (3-5 months) and the small size of the olive mills (Borja et al., 2006).
